Looking at the owners manual and the back of the unit, I don't think you can bridge the GFA5500. Seems to be (2) channel stereo only. Maybe they could be sent in to rewire to mono, but they are not user switchable.
Mono Amps
I own a Adcom GFA5500 and interested in bridging to mono. If I get another 5500 can they be bridged as mono?